1. Field of the Invention
The present invention relates to an image forming apparatus which forms a toner image on a photoconductive drum according to a photoelectrostatic process and more particularly to an image forming apparatus equipped with a pre-transfer drum charger which charges a photoconductive drum before transferring a toner image on a paper.
2. Description of the Related Art
An image forming apparatus using a photoelectrostatic process, for instance, an electrostatic copying machine contains an image reading portion to read an image to be copied and an image forming portion to form an image to be copied based on an image read by way of this image reading portion and output the formed image on a transfer material. This image forming portion has a paper conveying path to take a transfer material, that is, paper out of a paper stacker one by one and convey a paper having a transferred and fixed image to an exit port.
In recent years, an electrostatic copying machine equipped with a pre-transfer drum charger which charges a photoconductive drum before transferring a toner image formed on the photoconductive drum through the electrostatic process has been put for practical use in its image forming portion as described above. This pre-transfer drum charger is arranged around a photoconductive drum and between a developing device and a transferring device.
This pre-transfer drum charger charges an toner image formed on a photoconductive drum using AC voltage, DC voltage having the same polarity as that of a toner or AC voltage with DC voltage superposed.
Thus, as a result of the charging of a toner image by the pre-transfer drum charger, it becomes possible to discharge the surface of the photoconductive drum after the development and at the same time, enhance the electric charge retained by the toner image and efficiently transfer the toner image in the next transfer process and the separation process. Further, it also becomes possible to prevent occurrences of so-called re-transfer phenomenon wherein a toner to be transferred on a paper is returned to the photoconductive drum, image void, etc.
However, such a conventional pre-transfer drum charger has such problems as described below.
That is, as shown in FIG. 1, a developing device 33 has normally a small amount of a reverse charged toner B which has the polarity reverse to a normal toner A besides this normal toner A which has the positive polarity and is adsorbed to an image portion of a photoconductive drum 30 around a developing roller 33a.
The potential of the white ground portion, that is, the non-image forming portion of the photoconductive drum 30 is at, for instance, -30 through -150 V. On the other hand, the developing roller 33a of the developing device 33 is set at a higher potential than that of the white ground portion of the photoconductive drum, for instance, at -200V.
So, static electricity is added to the normal toner A charged with positive electric charge in the arrow direction shown in FIG. 1 and the toner A is attracted to the developing roller 33a having the higher negative potential. Thus, the white ground portion of the photoconductive drum 30 is not developed.
On the other hand, static electricity is added to the reverse charged toner B charged with negative electric charge in the arrow direction in FIG. 1 and the toner is attracted to the photoconductive drum 30 side which has the lower negative potential. Thus, the toner is adhered to the white ground portion of the photoconductive drum 30.
On a conventional pre-transfer drum charger, there are such problems that electric charge of the same polarity as the normal toner is uniformly given to the reverse charged toner B adhered on the non-image portion of the photoconductive drum in addition to the normal toner A adhered on the image portion of the photoconductive drum. As a result, the reverse charged toner B is also transferred on a paper in the next transferring process and the so-called fog is produced, deteriorating the quality of an image formed on a paper.
Further, in a developer comprising two ingredients having toner to develop an electrostatic latent image on a photoconductive drum and a carrier to feed these toners to a developing region, if the toner content increased from the 5 to 6% proper toner mixed ratio, that is, a toner concentration increased, a reverse charged toner content also increases. As a result, the fog level on a paper increases and the quality of image will be extremely deteriorated.